Sterically Restricted Tin Phosphines, Stabilized by Weak Intramolecular Donor–Acceptor Interactions

Abstract

(4)). The degree of intramolecular P-Sn bonding within the series was investigated by X-ray crystallography, solution and solid-state NMR spectroscopy and density functional theory (DFT/B3LYP/SBKJC/PCM) calculations. All members of the series adopt a conformation such that the phosphorus lone-pair is located directly opposite the tin centre, promoting an intramolecular donor-acceptor P→Sn type interaction. The extent of covalent bonding between Sn and P is found to be much greater in triorganotin chlorides 2-4 compared with triphenyl derivative 1. Coordination of a highly electronegative chlorine atom naturally increases the Lewis acidity of the tin centre, enhancing the lp(P)−σ*(Sn−Y) donoracceptor 3c-4e type interaction, as indicated by conspicuously short Sn-P peri-distances and significant 1 J( 31 P, 119 Sn) spin-spin coupling constants (SSCCs) in the range 740-754 Hz. Evidence supporting the presence of this interaction was also found in solidstate NMR spectra of some of the compounds which exhibit an indirect spin-spin coupling on the same order of magnitude as observed in solution. DFT calculations confirm the increased covalent bonding between P and Sn in 2-4, with notable WBIs of ca. 0.35 obtained, compared to 0.1 in 1.

Synthetic, Structural, and Spectroscopic Studies of Sterically Crowded Tin–Chalcogen Acenaphthenes

“…12 As such, much stronger multicentre bonding is predicted to occur in 5-9 as a consequence of the increased Lewis-acidity at the tin centre, explaining the notable increase in J( 119 Sn, 77 Se) through-space coupling observed in chlorides 6 (163 Hz) and 8 (142 Hz) compared to 2 (68 Hz). This is consistent with the findings from previous studies of phosphorustin compounds, 33,34 where substantial WBIs of up to 0.36 were calculated for a series of analogous triorganotin chlorides in which the Sn-P peri-distances were also notably compressed (72% ∑r vdW ). 33 The presence of a hypervalent 3c-4e type interaction operating in 5-9 is further supported by the natural reduction in the Sn-Cl bond order observed following donation of the chalcogen lone-pair into the σ*(Sn-Cl) orbital, with the equatorial SnCl bond in each case becoming weaker and longer than in the corresponding triorganotin chloride starting material [ As expected, the stronger 3c-4e Cl-Sn-E bonding present in 5-9 has a greater impact on counteracting the steric repulsion that invariably builds up between the closely aligned Sn and E lonepair orbitals and is thus accompanied by a substantial reduction in molecular distortion.…”

“…This is consistent with the findings from previous studies of phosphorustin compounds, 33,34 where substantial WBIs of up to 0.36 were calculated for a series of analogous triorganotin chlorides in which the Sn-P peri-distances were also notably compressed (72% ∑r vdW ). 33 The presence of a hypervalent 3c-4e type interaction operating in 5-9 is further supported by the natural reduction in the Sn-Cl bond order observed following donation of the chalcogen lone-pair into the σ*(Sn-Cl) orbital, with the equatorial SnCl bond in each case becoming weaker and longer than in the corresponding triorganotin chloride starting material [ As expected, the stronger 3c-4e Cl-Sn-E bonding present in 5-9 has a greater impact on counteracting the steric repulsion that invariably builds up between the closely aligned Sn and E lonepair orbitals and is thus accompanied by a substantial reduction in molecular distortion. This is most apparent within the acenaphthene plane where the divergence of the exocyclic Sn-C Acenap and E-C Acenap bonds is notably less pronounced [splay angles 5 10.9°[11.3°]; 6 12.6°; 7 10.8°; 8 12.7°; 9 9.8°] than in the triphenyltin derivatives 1-4 [splay angles 16-21°] (Figures 5 and 6).…”

“…This leaves the tin centre coordinated to two carbon atoms, one from each of the coupling acenaphthene backbones, with a C-Sn-C bond angle of 151.4(3)°. Alternatively, the two Sn-Cl bonds can be thought to retain enough covalent character as to describe the geometry as distorted disphenoidal (see-saw), similar to the related tin-bromine dichloride derivative {AcenapBr} 2 SnCl 2 , 32 and with the four C-Sn-Cl bond angles summing to 397°. The two additional intramolecular S→Sn lone-pair interactions, which complete a quasi-octahedral geometry around the tin centre, thus play an important role in determining the overall conformation of the metal.…”

“…For instance, the increased Lewis acidity of the tin centre in triorganotin chlorides O naturally enhances the lp(P)−σ*(Sn−Y) donor−acceptor 3c-4e type interaction (WBI 0.32-0.36), resulting in conspicuously short P•••Sn distances up to 29% shorter than the sum of van der Waals radii. 33 In comparison, separations in triphenyltin analogues (N) are notably longer (82-84% ∑r vdW ), but a significant degree of covalency is still predicted with WBIs of ~0.13 predicted for the P→Sn interaction. 33 In this work we describe the synthesis and characterization of a series of related mixed chalcogen-tin peri-substituted acenaphthenes [Acenap(SnPh 3 )(ER), Acenap = acenaphthene-5,6-diyl, ER = SPh (1), SePh (2), TePh (3), SEt (4); Acenap(SnPh 2 Cl)(EPh), E = S (5), Se (6); Acenap(SnBu 2 Cl)(ER), ER = SPh(7), SePh (8), SEt (9); {Acenap(SPh 2 )} 2 SnX 2 , X = Cl (10), Ph (11); Figure 2] and investigate how the substituents bound to both the tin and chalcogen centers affect the strength of the E→Sn donor-acceptor interaction.…”

“…During a study of sterically encumbered bromine-tin derivatives (L, M), 32 and more recently in a series of phosphorustin compounds (N-P), 33,34 the strength of the intramolecular Y→Sn donor-acceptor interaction was found to be highly dependent on the electronics of the substituents bound to Sn. In both series, the extent of covalent bonding was much greater in triorganotin chlorides in comparison to their trialkyltin analogues.…”